1. Field of the Invention
This invention relates to methods and materials for manufacturing a specialized type of plate and fin type heat exchanger and, in particular, to a method and materials for manufacturing a gas turbine regenerator heat exchanger.
2. Description of the Prior Art
The present invention deals with a particular type of plate and fin heat exchanger known in the relevant arts as the “gas turbine regenerator.” This type of heat exchanger has been developed for use with large gas turbines for improving turbine efficiency and performance while reducing operating costs. Heat exchangers of the type under discussion are typically referred to as either “recuperators” or as “regenerators.” One typical application of such units is in conjunction with gas turbines employed in gas pipe line compressor drive systems.
In the typical gas turbine power plant application, the regenerator is used to heat compressor discharge air prior to its entry into the combustion chambers, thereby reducing the amount of fuel necessary to bring the combustion gases to the required operating temperatures. Heat is transferred to the compressor discharge air from hot turbine exhaust gases which pass through the regenerator in heat transfer relation with the compressor discharge air. The regenerator includes alternating stacked air and gas channels of the plate-fin type to effect the heat transfer.
Gas turbine regenerators of the type under consideration have included box-like structures having plate-fin tube banks with the entire regenerator banded together by tie straps which interconnected structural end frames. Compressor discharge air, at the relatively high operating pressures encountered, tends to warp or bow the end frame structures of these devices, thereby presenting a point of potential material failure. Also, the design of the prior art units have, to some extent, been limited in their recommended operating temperature ranges by virtue of the materials employed in their fabrication as well as by the fabricating techniques which were employed.
For example, the previously used compression-fin designs at times developed unbalanced internal pressure-area forces in a regenerator of suitable size. Unbalanced forces of this type tended to split the regenerator core structure apart during operation. More recently, technology has advanced so that the internal pressure forces are more evenly balanced. However, even with the advances which have been made in materials and manufacturing techniques, the changes in dimension of the overall unit due to thermal expansion and contraction become significant and must be taken into account in the overall design. These thermal size changes must be accommodated in some fashion to prolong the useful life of the regenerator. The problem is exaggerated by the fact that the regenerator must withstand a lifetime of thousands of heating and cooling cycles due to the operating mode of the associated turbo-compressor which is often started and stopped repeatedly.
U.S. Pat. No. 3,866,674, issued Feb. 18, 1975, assigned to General Electric Company, shows a regenerator design which is typical of the prior art in that the plate and fin tube banks were joined at either of two opposite ends to a cylindrical inlet and outlet plenum, respectively. The air inlet and outlet plenums were formed with semi-circular slotted openings disposed along the longitudinal axis of each plenum. The pressure tubes making up the tube banks also had semi-circular end regions which were received within the openings in the plenums where they were welded in place. The junctions between the tube sheets and cylindrical plenums presented potential failure points in the design when subjected to the extreme temperature and pressure conditions discussed above.
U.S. Pat. No. 4,229,868, issued Oct. 28, 1980, assigned to The Garrett Corporation, was an improvement on the above plenum and tube sheet design. This regenerator was constructed of a plurality of formed plates and fins brazed together into a complete unit comprising manifolds and a heat exchanging core in a single counter-flow device. The respective end portions of the heat exchanger plates are formed with a peripheral flange which, when joined with the corresponding flange of an adjacent formed tube plate, provides a boundary seal for containing the air fin passages provided by the thus-joined pair of heat exchanger plates. Each end portion of the formed tube plate had an opening encircled by a collar portion, thus defining a manifold section through the plate. The collar portion was cut back along the side facing the core portion so as to provide communication between the manifold section and the air fin passages. The formed tube plate also had a ring offset from the plane of the plate and extending about the manifold opening. This ring had a flat base portion which served to provide spacing between the joined plates for the gas fin passages and to seal the manifold sections of the joined heat exchanger plates from the gas passages.
Rising fuel costs in recent years have dictated that gas turbine power plants operate with increased thermal efficiency, and new operating methods require a regenerator that will operate more efficiently at higher temperatures while possessing the capability of withstanding thousands of starting and stopping cycles without leakage or excessive maintenance costs. As a result, a need continues to exist for improvements to the regenerator designs which are used with gas turbines employed in gas pipe line compressor drive systems, as well as in other industrial applications.
A need continues to exist for an improved regenerator design in which potential weak points which would be subject to rupture from internal pressure forces are eliminated.
A need also exists for such an improved design which features a brazed, stainless steel core which allows for greater efficiency and ultimately higher cost-savings than other types of regenerators currently in the marketplace.